In golf balls commercially selling, there are solid golf balls such as two-piece golf ball, three-piece golf ball and the like, and thread wound golf balls. Recently, the two-piece golf ball and three-piece golf ball, of which flight distance can be improved while maintaining soft and good shot feel at the time of hitting as good as the conventional thread wound golf ball, generally occupy the greater part of the golf ball market. Multi-piece golf balls represented by the three-piece golf ball have good shot feel while maintaining excellent flight performance, because they can vary hardness distribution, when compared with the two-piece golf ball.
The golf balls comprising a two-piece core, formed by placing an intermediate layer between the core and the cover of the two-piece solid golf ball, are suggested in Japanese Patent Kokai publication Nos. 239068/1997, 313643/1997, 107327/2000, 271249/2000, 296187/2000, 300695/2000 and the like. In the golf balls, it is attempted to improve both the flight performance and shot feel by using thermoplastic resin, such as polyurethane-based thermoplastic elastomer, ionomer resin or mixtures thereof, for the intermediate layer, and adjusting the hardness or hardness distribution, deformation amount, specific gravity and the like of the core, intermediate layer and cover.
In Japanese Patent Kokai Publication No. 239068/1997, a three-piece solid golf ball comprising a core, an intermediate layer and a cover is described. The core has a central point hardness in JIS-C hardness of not more than 75 and a surface hardness in JIS-C hardness of not more than 85, the surface hardness of the core is higher than the central point hardness of the core by 8 to 20, the hardness of the intermediate layer is higher than that of the surface of the core by not less than 5. The hardness of the cover is lower than that of the intermediate layer by not less than 5, and a ratio of dimple area to surface area of the golf ball is not less than 62%.
In Japanese Patent Kokai Publication No. 313643/1997, a three-piece solid golf ball, of which an intermediate layer is placed between a core and a cover, is described. The core has a central point hardness in JIS-C hardness of not more than 75 and has a surface hardness in JIS-C hardness of not more than 85, the surface hardness is higher than the central point hardness by 5 to 25, a hardness of the intermediate layer is higher than the surface hardness of the core by less than 10, and a hardness of the cover is higher than the hardness of the intermediate layer. In the two above golf balls, since the hardness difference in the core is small, the deformation amount at the time of hitting is small and spin amount is increased, and the flight performance is not sufficiently obtained.
In Japanese Patent Kokai publication No. 107327/2000, a three-piece solid golf ball comprising a core composed of a center having a diameter 27 to 37 mm and an intermediate layer covering the center, and a cover covering the core. The specific gravity of the center (a) is smaller than that of the intermediate layer (b), a surface hardness in JIS-C hardness of the center (Y) is higher than a central point hardness in JIS-C hardness of the center (X) by not less than 8, a surface hardness of the core (Z) is not less than 80, a difference (p−q) between a deformation amount of the center (p) and that of the core (q), when applying from an initial load of 98 N to a final load of 1275 N, is not less than 5, and Shore D hardness of the cover is not more than 60.
In Japanese Patent Kokai publication No. 271249/2000, a three-piece solid golf ball comprising a core composed of an inner core and an outer core, and at least one layer of an cover formed on the core. The inner core has a diameter of 30 to 39.5 mm and a central point hardness in JIS-C hardness of 55 to 70, a JIS-C hardness at the distance of 15 mm from the center point is higher than the central point hardness by 5 to 20, the inner core is formed from heat molded article of rubber composition comprising polybutadiene, co-crosslinking agent, organic peroxide and filler as an essential component; the outer core has a thickness of 0.3 to 2.0 mm and a surface hardness in JIS-C hardness of 75 to 90, the surface hardness is higher than the central point hardness of the inner core by 10 to 35, the outer core is formed from heat molded article of rubber composition, the cover comprises thermoplastic resin as a base resin; and the outmost layer of the cover has a thickness of 1.5 to 2.5 mm and surface hardness in Shore D hardness of 64 to 72. In the two above golf balls, since the intermediate layer is formed from rubber composition, the durability is not sufficiently obtained.
In Japanese Patent Kokai publication No. 296187/2000, a three-piece solid golf ball comprising a center, an intermediate layer formed on the center, and an outer layer covering the intermediate layer. The center has a central point hardness in JIS-C hardness of 55 to 75 and a surface hardness in JIS-C hardness of 65 to 85, and the surface hardness of the center is higher than the central point hardness of the center by 10 to 20, a JIS-C hardness of the intermediate layer is higher than the surface hardness of the center by 10 to 20, and a JIS-C hardness of the outer layer is higher than the hardness of the intermediate layer by 5 to 15. In the golf ball, since the hardness difference in the core is small, the deformation amount at the time of hitting is small and the spin amount is increased, and the flight performance is not sufficiently obtained.
In Japanese Patent Kokai publication No. 300695/2000, a three-piece solid golf ball comprising a core composed of an inner core and an outer core, and at least one layer of an cover formed on the core. The inner core has a diameter of 30 to 40.4 mm and a surface hardness in JIS-C hardness of 60 to 85, a central point hardness in JIS-C hardness of the inner core is lower than the surface hardness by 5 to 30; the outer core has a thickness of 0.2 to 1.3 mm, and a surface hardness in JIS-C hardness of the outer core is lower than the surface of the inner core by 2 to 30. In the golf ball, since the surface hardness of the outer core is lower than the surface hardness of the inner core, the flight performance is not sufficiently obtained.
Many conventional golf balls other than the above golf balls have been proposed. However, in the golf balls, its hardness difference in the core is not optimized, and golf balls having both excellent flight performance and good shot feel, and having the durability for practical use, have not been sufficiently obtained. Therefore, it is required to provide a golf ball having better shot feel, better flight performance and better durability.